White Topknot

It’s winter on Mars, too. Earth’s winter solstice was Dec. 21; Mars’s was today, Jan. 11, at about 8 hours Universal (Greenwich) Time.

We’re talking, with our usual north-hemisphere chauvinism, of the moments when the planets’ north poles are tipped most away from the Sun, so that their northern hemispheres are receiving lowest and briefest sunlight. For their southern hemispheres it’s summer.

Mar and NEarth
Mars beside Earth; from the new edition of our book for the young, To Know the Stars.

Yes, Mars is Earth’s smaller brother in some ways. To quote from the Mars section of Astronomical Calendar 2015, “Earth and Mars are curiously similar in rotation: they spin in 24 and 24-1/2 hours, respectively, around polar axes that are tilted 23.44° and 25.19° to the planes in which they travel.”

And now we find Mars’s winter solstice occurring around the same time as Earth’s. Does this mean that these events are somehow linked?

No, because the four season-marking events of a planet – the two solstices and the two equinoxes – have to be at quarters of the way around its orbit, and Earth’s orbit takes 365 days whereas Mars’s takes 687 (Earth) days. To list a couple of Mars cycles:

2014 Feb 15 summer solstice
2014 Aug 17 autumn equinox
2015 Jan 11 winter solstice
2015 Jun 18 spring equinox
2016 Jan 3 summer solstice
2016 Jul 4 autumn equinox
2016 Nov 28 winter solstice
2017 May 5 spring equinox

Mars is now fairly low in the evening sky (38 degrees east of the Sun), daily getting lower as it sinks slowly toward its “superior conjunction” behind the Sun on June 14. It progressed from Capricornus into Aquarius yesterday, and stands about 20 degrees above Venus and Mercury. Take another look at our evening-horizon-scene diagram for yesterday’s story called “Quasi” (I hope you can see it at the top of the list on the right). Why use more space on the server for yet another diagram which will be hardly any different! I heard something scary about the heat generated by “server farms,” which must be increased a little by every byte we upload to them. (Comment is invited from anyone who knows better.)

Because Mars is toward the opposite side of its orbit from us, it is about 2 Sun-Earth distances (astronomical units) away, thus shining relatively dimly (at magnitude 1.1, about the same brightness as the stars Pollux or Fomalhaut). Its apparent disk has shrunk to a bit less than 5 seconds wide.

So in your binoculars or telescope it will be possible, but not easy, to glimpse Mars’s little rust-colored face, and to discern its white scalp, the north-polar icecap, nodding toward you. A bit like Tintin’s quiff. I’d better not get into a digression about Tintin. Yet.

The cap is made of water ice, with a topping of carbon dioxide frost which later, when warmed, will blow away off it, creating strong winds. Like Tintin’s quiff, the northern icecap is not very large. Because Mars’s orbit is much more eccentric than Earth’s, and Mars’s northern winter occurs in the part of the orbit that is nearer to the Sun, it is a mild winter. In the southern winter of January 2016, the south-polar icecap will show (though in our morning sky) and will be much larger – a white beard.

Mars’s northern winter is mild – for Mars. Being one and a half times farther from the Sun than we are, and with a thin atmosphere, the little planet has an average temperature about 78 degrees Celsius (140 Fahrenhait) colder than ours. An article in the January issue of Scientific American, “Better than Earth,” about the possibility of planets even more hospitable to life than ours, points out that the “habitable zone” around the Sun – where surface water can exist in the liquid state – moves gradually outward and Earth is already toward the inner edge of it. So in a remote future (though long before both get incinerated by the red giant Sun) Earth may be too hot for life. And Mars may be in the habitable zone, if it can get back the water it lost.


6 thoughts on “White Topknot”

    1. Mars’s southern summer is warmer only in the sense that it is warmer than Mars’s northern summer. All Mars seasons are far colder than anything on Earth.
      In the far future when the “habitable zone” moves outward (because the Sun is getting hotter), it will be warmer on Mars, but it will also be far too hot on Earth!
      Daylight lengthens on the northern hemisphere of the Earth, from the December solstice onward, because the hemisphere gradually nods more toward the Sun. Therefore New York (for instance) spends longer traveling around from the sunrise side to the sunset side.

  1. I can’t even pretend to understand the math behind this, but your illustration is beautiful.

    Would love to hear your take on how it’s “warmer on Mars than on Earth,” and, unrelatedly (is that a word?) about how the length of daylight accelerates as the Earth’s pole swings back.

  2. Question: If, in a chauvenistic north hemispheric sense, it is presently winter on Mars, then wouldn’t it be the white groin patch of the southern polar cap from its below-the-belt nether regions rather than the north polar cap that is currently tilting our way?

    1. Welcome. Curtis. You’re right: in the middle of the planet’s north-hemisphere winter, its north pole is tipped maximally away from the Sun. And, approximately, from us, because Mars is roughly in the same direction from us as the Sun.
      My drawing seems to show Mars in northern summer, with the large southern icecap almost out of sight.

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